Compensation for independent clocks in relayed communication over packet-based networks

Abstract

Methods, apparatuses, and systems are presented for compensating for independent clocks in relayed modem communications involving receiving data packets from a first network device, forwarded over a packet-based network, at a second network device, the data packets representing data in a first modulated signal from a first modem, the first modulated signal being associated with a first symbol rate, placing data derived from the data packets into a queue in the second network device, generating a second modulated signal at the second network device for transmission to a second modem, the second modulated signal being generated using data retrieved from the queue and being associated with a second symbol rate, obtaining at least one queue size measurement associated with data in the queue, and effectively adjusting the second symbol rate based on at least one queue size measurement associated with data placed in the queue in the second network device.

Description

BACKGROUND OF THE INVENTION[0001]FIG. 1 depicts an illustrative environment in which relayed modem communication is carried out over a packet-based network. As shown in the figure, a modem 102 is connected to network device 104, which is in turn connected to a packet-based network 106. Also connected to the packet-based network 106 is another network device 108, which in turn is connected to another modem 110.[0002]Each of the modems 102 and 110 is capable of generating and receiving modulated signals that convey digital information. A wide range of modulation schemes may be implemented. These include different types of amplitude modulation such as quadrature amplitude modulation (QAM), different types of phase shift keying modulation such as quadrature phase shift keying modulation (QPSK), different types of frequency shift keying modulation (FSK), etc. The appropriate type of modulation scheme or schemes used may depend on the specific needs of a particular implementation. Some ex...

AI Technical Summary

Benefits of technology

[0014]The present invention relates to methods, apparatuses, and systems for compensating for independent clocks in relayed modem communications involving (1) receiving data packets from a first network device, forwarded over a packet-based network, at a second network device, the data packets representing data transmitted in a first modulated signal from a first modem to the first network device, the first modulated signal including symbols associated with a first symbol rate based on a first clock, (2) placing data derived from the data packets into a queue in the second network device, (3) generating a second modulated signal at the second network device for transmission to a second modem, the second modulated signal being generated using data retrieved from the queue, the second modulated signal including symbols associated with a second symbol rate based on a second clock, (4) obtaining at least one queue size measurement associated with data placed in the queue in the second network device, and (5) effectively adjusting the second symbol rate, based on at least one queue size measurement associated with data placed in the queue in the second network device.

[0018]The at least one queue size measurement may corresponds to at least one measured size of a buffer. The second symbol rate may be effectively adjusted by introducing a phase adjustment in the second modulated signal. An adjustment made to the second symbol rate may be restricted to a predetermined maximum limit. Upon occurrence of a known event, the at least one queue size measurement may be cleared and re-obtained to effectively adjust the second symbol rate.

Problems solved by technology

Even if the clock signals are adjusted to be nominally the same rate and are only slightly different, the resulting difference between the symbol rates can accumulate over time, which is sometimes referred to as “clock drift.” This eventually leads to data underrun or data backup.

Just as an example, in an system carrying voice data, a data back up condition may cause noticeable audio delay.

This again can lead to data underrun or data backup conditions.

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